The normal or " triplet" ground state (%g-) of molecular oxygen is that of a biradical, -0-0.. but " singlet " oxygen (lAg) O=O, which is structurally similar to ethylene, is the initial product of heterolytic decompositions of hydrogen peroxide or of per-acids. " Singlet " oxygen reverts to normal " triplet " oxygen by a bimolecular reaction that produces chemiluminescence but is sufficiently long-lived to be able to react as a dienophil; it combines with anthracenoid hydrocarbons at their meso positions, or adds to suitable 1.3-dienes.Singlet oxygen has been produced (i) by the reaction of alkaline hydrogen peroxide with either sodium hypochlorite or bromine and (ii) by decompositions of alkaline solutions of per-acids, either alone or with added hydrogen peroxide. By these methods the endo-peroxides (the " photoperoxides ") of a number of 9.1 O-disubstituted anthracenes have been prepared in the dark, whilst 9-substituted anthracenes have given typical hydrolysis products of the primary endo-peroxides. The most effective route uses the H202-Br2 reaction, but gives side-products owing to concurrent bromine addition unless a two-phase reaction mixture is used. With the H202-NaOCI system the dienophilic addition of oxygen to lumisteryl acetate and to 2.4-cholestadiene has also been established.Similar evidence is given to show that the chemiluminescent Trautz reaction also yields singlet oxygen.SPECTROSCOPISTS have shown that molecular oxygen ated,l contains no unpaired electron. These allotropes can exist for finite periods of time in a number of different of oxygen differ chemically as well as physically. Thus electronic states. Of these the ground state (3Xg-) is ground-state, or "triplet," oxygen can rationally be paramagnetic, having two unpaired electrons with written as *O-Oa because it is a stable biradical; its parallel spins in degenerate V , orbitals, whilst the more chemical reactions are of free-radical type since they energy-rich lag state, with which the visible lumin-normally proceed by chain mechanisms whilst again escence of excited oxygen molecules is probably associ-the vast majority of the reactions of molecular oxygen September 1965.
When phthalein dyes are oxidised by alkaline hydrogen peroxide within the cavity of an e.s.r. spectrometer then the spectra of substituted p-benzosemiquinone radicals can be observed. This shows that oxidation occurs by HOzattack and Dakin rearrangement to break off one phenolic ring as a p-benzoquinol, leaving a substituted benzophenone.
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